The present invention relates to an umbilical for use in the offshore production of hydrocarbons, and in particular to a power umbilical for use in deep water applications.
An umbilical consists of a group of one or more types of elongated active umbilical elements, such as electrical cables, optical fibre cables, steel pipes and/or hoses, cabled together for flexibility, over-sheathed and, when applicable, armoured for mechanical strength. Umbilicals are typically used for transmitting power, signals and fluids (for example for fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.
The umbilical cross-section is generally circular, with the elongated elements being wound together either in a helical or in a S/Z pattern. In order to fill the interstitial voids between the various umbilical elements and obtain the desired configuration, filler components may be included within the voids.
The API (American Petroleum Institute) 17E “Specification for Subsea Umbilicals” provides standards for the design and manufacture of such umbilicals.
Subsea umbilicals are being installed at increasing water depths, commonly deeper than 2000 m. Such umbilicals have to be able to withstand the increasingly severe loading conditions during their installation and their service life.
The main load bearing components in charge of withstanding the axial loads caused by the weight and movements of the umbilical are steel pipes (see U.S. Pat. No. 6,472,614, WO93/17176 and GB2316990), steel rods (see U.S. Pat. No. 6,472,614), composite rods (see WO2005/124213), or tensile armour layers (see FIG. 1 of U.S. Pat. No. 6,472,614).
The other elements, i.e. the electrical and optical cables, the thermoplastic hoses, the polymeric external sheath and any polymeric filler components, do not contribute significantly to the tensile strength of the umbilical.
Electrical cables used in subsea umbilicals fall into two distinct categories respectively known as power cables and signal cables.
Power cables are used for transmitting high electrical power (typically a few MW) to powerful subsea equipment such as pumps. Power cables are generally rated at a medium voltage between 6 kV and 35 kV. A typical prior art power cable is illustrated in FIG. 1 of the accompanying drawings. From the inside to the outside, it comprises a central copper conductor 2a, three semi-conductor and electrical insulation layers 2b, a metallic foil screen 2c and an external polymeric sheath 2d. The central conductor 2a generally has a stranded construction and a large section typically comprised between 50 mm2 and 400 mm2. Three phase power can be provided by three such cables bundled together within the umbilical structure.
Signal cables are generally used for transmitting signals and low power (<1 kW) to electrical devices on the seabed. Signal cables are generally rated at a voltage smaller than 3000V, and typically smaller than 1000V. Signal cables generally consist of small section insulated conductors bundled together as pairs (2), quads (4) or, very rarely, any other number, said bundle being further over-sheathed.
An example of a prior art quad signal cable is illustrated in FIG. 2 of the accompanying drawings. FIG. 2 shows four small size stranded copper conductors 3a individually over-sheathed by polymeric insulation layers 3b and helically bundled together. A polymeric filler material 3c is added to fill the voids in the bundle and to achieve a cylindrical shape. This arrangement is optionally surrounded by an electromagnetic shielding 3g made from a wrapped copper or aluminium foil. A polymeric external sheath 3d protects the cable against mechanical damage and water ingress.
The copper conductors of electrical cables are not load-bearing components, because of the low tensile strength of copper. These copper conductors effectively only add weight to the umbilical. Unless protected, these electrical conductors may therefore be damaged by excessive elongation or crushing, especially under severe conditions such as in deep water and/or in dynamic umbilicals.